How can I blend two images?
Using cycles you can use a mixRGB node to combine two images into one, each image can use a different uvmap. The Fac of the mixRGB defines how much of each image is visible.

You can then use a third greyscale image to define the amount that the two images mix together, white showing one texture and black the other and 50% grey being an even mix of both, texture painting can be useful to position the transition where you want it.